1. Field of the Invention
The present invention pertains to an aqueous inkjet ink made from certain titanium dioxide slurries and an associated inkjet ink set for inkjet printing. The invention also pertains to a method of inkjet printing with the ink and ink set. The use of the specific titanium dioxide slurries described herein result in inkjet inks having improved stability and better anti-settling performance with less pigment agglomeration and flocculation over time.
2. Description of the Related Art
Inkjet printing is a non-impact printing process in which droplets of ink are deposited on print media, such as paper or polymeric substrates, to form the desired image. The droplets are ejected from a printhead in response to electrical signals generated by a microprocessor.
Colored inkjet inks comprise one or more colorants that are dissolved (e.g., dyes) and/or dispersed (e.g., pigments and dispersed dyes) in the ink vehicle. The ink vehicle can be aqueous (a significant amount of water) or non-aqueous (predominantly organic liquid), and the ink is referred to as aqueous or non-aqueous ink accordingly.
Aqueous ink is advantageous because water is especially environmentally friendly. There are many applications, though, where aqueous ink is typically unsuitable and non-aqueous ink must be used. Many, if not most of these non-aqueous ink applications involve printed articles on hydrophobic substrates, and particularly printed articles on polymer substrates, which will be exposed to sunlight, and the preferred colorants in these applications are pigments because of their well-know advantage in fade resistance compared to dyes.
Dispersion of pigments in a non-aqueous vehicle is substantially different than dispersion in an aqueous vehicle. Generally, pigments that can be dispersed well in water do not disperse well in non-aqueous solvent, and vice versa.
Also, the demands of inkjet printing are quite rigorous and the standards of dispersion quality are high. Thus, pigments that may be “well dispersed” for other applications are often still inadequately dispersed for inkjet applications.
There is a need for improved pigment selection especially for a stable aqueous ink for inkjet inks. In particular, there is a need for white pigments that can be sufficiently stabilized in inkjet compatible formulations so that the resultant ink can be effectively jetted, even after being stored or otherwise unused for some period of time. In addition the ability to use an ink containing a white pigment to complement other inks of an ink set can lead to improved images, especially when lighter tones and/or higher degrees of coverage or opacity are needed.
White inks are useful and provide good visibility when printed on trans-parent and colored surfaces. White printing on these surfaces is desirable in numerous end uses, such as the computer industry (printed circuit boards, computer chips), recording industry (tapes, film, etc.), packaging and automotive coatings. White ink may be used not only to detail and add decals to automobiles, but also to other motor vehicles, including trucks, planes and trains, as well as bicycles, etc. White ink may also be useful on other surfaces, such as plastics, wood, metal, glass, textiles, polymeric films and leather for both practical and ornamental purposes.
White ink formulations typically contain a particulate white pigment dispersed in a solvent (aqueous or non-aqueous)/resin system. Current white ink formulations are not acceptable for numerous applications, such as commercial inkjet applications, primarily because of poor stability resulting in pigment settling and agglomeration. Poor stability may result in “nozzle outs” or plugging of the ink jet nozzles. For example, a typical print head on an industrial printer has 256 nozzles, each nozzle head having a diameter of about 50 microns in size. Large pigment particles and agglomerates may plug the nozzles. Poor stability also results in poor hiding, non-uniform coverage and poor clarity in the printed surface.
White ink formulations based on inorganic white pigments, such as titanium dioxide (TiO2), may fail because of poor stabilization of the TiO2 pigment. Pigment agglomeration and flocculation are often at fault in poor performance of white inks, particularly White inkjet inks, due to settling and nozzle plugging problems.
As a result, there is a need for an ink formulation containing a white pigment for use in ink and inkjet systems that avoid the aforementioned negative attributes. There is further a need for a titanium dioxide slurry that does not suffer from stability problems when incorporated into ink formulations. There is still a further need for an aqueous system that includes all these benefits. The present invention meets these needs.